With the development of communications technologies, owing to advantages such as high bandwidth, good scalability, use of fewer feeder fibers, and a wide coverage area, a PON (PON) system has been widely applied in the broadband access field.
As shown in FIG. 1A, a PON system generally includes: an OLT (optical line terminal), an ONU (optical network unit)/ONT (optical network terminal), and an ODN (optical distribution network), where the ODN includes a splitter (a passive optical splitter), a feeder fiber, and a distribution fiber.
In a transmission process, the PON system uses a single-fiber bidirectional transmission mechanism to transmit two waves that are in opposite directions and have different wavelengths by using one fiber, where each wave carries a digital signal in one direction. In order to separate multiple users' signals in incoming and outgoing directions on a same fiber, the PON system uses the following two multiplexing technologies to transmit a digital signal: in a downlink transmission direction, refer to FIG. 1B; in an uplink transmission direction, refer to FIG. 1C.
With the development of communications technologies, in order to improve bandwidth, a TWDM (time/wavelength division multiplexing) PON system extended on a basis of the PON system emerges as the times require, specifically as shown in FIG. 1D. TWDM is an abbreviation of TDM (time division multiplexing) and WDM (wavelength division multiplexing). The TWDM-PON system and the PON system are the same in that an entire ODN structure remains unchanged, and a main difference is that a quantity of wavelength types corresponding to uplink and downlink light increases from one to more than two.
A process of transmitting a digital signal in a TWDM-PON system (an example in which a quantity of wavelength types corresponding to light increases from one to four is used) is as follows:
In a downlink transmission direction: Light corresponding to four different wavelengths is emitted by four lasers of an OLT respectively, enters a feeder fiber after passing through a multiplexer, and then arrives at an ONU. An optical receiver of the ONU only selects and receives light corresponding to one of the wavelengths, and therefore, a tunable filter needs to be disposed before the optical receiver. Because light corresponding to one of four wavelengths needs to be selected, four different filters may be prepared for different ONUs; or a tunable filter may be selected, and is configured for different wavelengths according to an actual need, thereby reducing a type of used filters.
In an uplink transmission direction: Any ONU may also emit light corresponding to one wavelength of four different wavelengths. In addition, the ONU may select four different lasers; or may use one laser, and adjust the laser to a specific wavelength according to a requirement, thereby reducing an ONU type. In uplink, light separately corresponding to the four different wavelengths arrives at a demultiplexer of an OLT after entering an ODN. The light separately corresponding to the four different wavelengths is split by the demultiplexer, and then enters different optical receivers.
In an actual application, in a TWDM-PON system, in order to reduce a size of an OLT module, reduce total power consumption, and improve port density of a line card, all lasers and multiplexers are integrated by using a photonic integration technology (including monolithic integration and hybrid integration) to form a miniaturized integrated optical transmitter, or all optical receivers and demultiplexers are integrated to form a miniaturized optical receiver. However, the foregoing optical transmitter or optical receiver has the following problem: When one laser of the optical transmitter is faulty, the entire optical transmitter needs to be replaced to ensure system performance. Similarly, if any optical receiver of the optical receiver is faulty, the entire optical receiver also needs to be replaced. Therefore, an integrated module in the TWDM-PON system has relatively low stability and relatively high operation costs.
In order to resolve the foregoing problem, in the prior art, some protection paths are added besides a path that works normally. An optical transmitter is used as an example. As shown in FIG. 1E, paths 1 to N are used for lasers working normally, where the lasers emit light of different wavelengths, and all of the emitted light is combined by using an optical multiplexer. 1 to M are used for lasers emitting light for protection. The light emitted from the paths 1 to N and the light emitted from the paths 1 to M for protection are combined by using a light combination device and are output from a common port at a right side. When the paths 1 to N work normally, the protection paths 1 to M are in a shutdown state, and output of the entire device is output of the light emitted from the paths 1 to N. When one path of the paths 1 to N is faulty, one path of the protection paths 1 to M is enabled to emit light. A wavelength of a signal and information on the path are completely consistent with light emitted from the faulty path. If multiple paths are faulty, multiple protection paths are enabled.
In the optical transmitter in the foregoing technical solution, multiple lasers configured to emit protective light need to be disposed, and therefore, complexity is relatively high. In addition, a light combination device used by the optical transmitter brings an extra loss, and all paths that work normally endure an extra power loss brought by the light combination device no matter whether a protection path works, lowering output optical power output optical power efficiency of an entire module.